What Makes Cookies Soft and Chewy or Cake Like, and What Makes Cookies Spread as They Bake - Alyce, here's some info

What makes cookies soft and chewy?
High moisture content does it. And that means the recipe, ingredients, baking time and temperature must be adjusted for the dough to retain moisture. Binding the water that is in butter, eggs, and brown sugar (it contains molasses, which is 10 percent water) with flour slows its evaporation. Also the dough needs a little extra flour to make it stiffer; Stiff dough spreads less, less liquid evaporates and so the cookies are thicker. Mass also helps cookies stay moist-big dollops of dough make softer and chewier cookies than tiny spoonfuls. Bake these thick cookies for a shorter time at a high temperature to firm them quickly and minimize spreading . Most important don't bake them too long-remove them from the oven when the cookie rim is brown and at least a third of the center top remains pale. The cooked centers will be soft.

Why are some cookies cake like instead of chewy?
A little extra liquid in the cookie dough from water, eggs, or milk makes the dough more elastic and adds steam as the cookies bake, making them puff more.

What makes a cookie crisp or crunchy?
Reducing the amount of ingredients that hold moisture-flour, eggs and brown sugar-makes it easy for liquid to evaporate, producing crisp cookies. The fat content is proportionately higher when other ingredients are cut back; the fat gets hotter than the water in the dough and drives out the moisture through evaporation. And since fat makes the dough softer and melts when hot, it makes the cookies spread. For crispness, bake cookies longer at lower temperature to give them more time to spread before they firm. Then bake them long enough to dry, and brown them evenly to develop the maximum toasty flavor and crisp texture throughout.

What else makes cookies spread as they bake?
Most often the culprit is low-fat butter or margarine spread, which has about 20 percent more water, used in place of regular butter or margarine. It's this extra liquid that causes the flattening. Low fat products can't be used interchangeable with regular fats for baking without recipe adjustments.
Cookies also spread when you drop high-fat dough onto a hot baking sheet; the heat melts the dough, and the cookies flatten before they're baked enough to hold their shape.

Source: Sunset Magazine, 1995